ADP5588 Data Sheet
Rev. C | Page 8 of 28
To prevent glitches or narrow press times registering as valid
key presses, the key scanner requires the key to be pressed for
two scan cycles. The key scanner has a sampling period of 25 ms,
so the key must be pressed and held for at least 25 ms to register
as pressed. If the key is continuously pressed, the key scanner
continues to sample every 25 ms. If a key that was pressed is
released for 25 ms or greater, the state machine sets the appro-
priate keys in the key event status register with the key pressed
bits cleared in the order detected. Because the release of a key is
not necessarily in sync with the key scan sampling period, it may
take between 25 ms and 50 ms for a key to register as released.
After the key is registered as released, the key scanner goes back
to idle mode. Figure 5 shows the row and column pins
connected to a typical 10 × 8, 80-switch keypad matrix.
KEYPAD SCAN AND DECODE
D0_PULL
J7
I7
H7
G7
F7
E7
D7
C7
B7
A7
J6
I6
H6
G6
F6
E6
D6
C6
B6
A6
J5
I5
H5
G5
F5
E5
D5
C5
B5
A5
J4
I4
H4
G4
F4
E4
D4
C4
B4
A4
J3
I3
H3
G3
F3
E3
D3
C3
B3
A3
J2
I2
H2
G2
F2
E2
D2
C2
B2
A2
J1
I1
H1
G1
F1
E1
D1
C1
B1
A1
J0
I0
H0
G0
F0
E0
D0
C0
B0
A0
R7 R6 R5 R4 R3 R2 R1
R0
C0 C1 C2 C3 C4 C5 C6 C7 C9C8
10 × 8 KEYPAD MATRIX
V
CC
D1_PULL
D2_PULL
D3_PULL
D4_PULL
D5_PULL
D6_PULL
D7_PULL
07673-010
Figure 5. Keypad Decode Configuration
Key Event Tracking
The 10-key event registers are set to act as a FIFO, meaning that
reading any of the 10-key event registers yields the key events in
the order they were pressed and released.
Tracking of key events is done with the help of the key event
counter (the KEC field in Register 0x03) and the FIFO/key
event registers (Register 0x04 through Register 0x0D). The KEC
count increases as keys are pressed and released; up to 10 events
can be logged in the counter. The FIFO/key event registers, on
the other hand, display the key events and their status (pressed
or released) as they are read out of the FIFO. The FIFO registers
are made of eight bits, with the MSB dedicated as the status bit
(1 indicates a press and 0 indicates a release); the remaining
seven bits are used to display binary representation of the keys
that are pressed or released.
The first read of any of the FIFO registers displays the first
event that happened and its status. Subsequent reads of the
same register replace the register data with the next event that
happens. If tracking of all the events is important, it is best to
used a single register per event. After all the events in the FIFO
are read, reading of any of the event registers yields a zero value.
Table 11 and Table 12 show the event sequences as they are
logged in and read from the FIFO. The 10 FIFO registers are
labeled A through J, and keys are labeled A0 through J7.
Table 11. Example of Event Sequence
Key Pressed/Released Status Key Event Counter
A0 Pressed 1
B1 Pressed 2
A0 Released 3
C2 Pressed 4
B1 Released 5
D3 Pressed 6
C2 Released 7
E4 Pressed 8
D3 Released 10
Table 12. Interpretation of FIFO Event Reading
Key Event
Counter
Key Event
Register
Read
Key Event Reg-
ister Content
(Binary)
1
Key Event
Register
Interpretation
10 N/A N/A N/A
9 D 1 0000001 Key A0 pressed
8 E 1 0001100 Key B1 pressed
6 F 1 0010111 Key C2 pressed
5 G 0 0001100 Key B1 released
4 A 1 0100010 Key D3 pressed
3 B 0 0010111 Key C2 released
2 H 1 0101101 Key E4 pressed
1 J 0 0101101 Key E4 released
0 I 0 0100010 Key D3 released
1
The first number indicates a key press or key release in Bit 7 of the key event
register: 1 = key press; 0 = key release.
Key Event Overflow
The ADP5588 is equipped with an overflow feature to handle
key events beyond the FIFO capacity. When all events are filled, any
additional events set the OVR_FLOW_INT bit in Register 0x02;
if the OVR_FLOW_IEN bit in Register 0x01 is set, the host
processor is also interrupted when overflow occurs. When the
FIFO is not full, new events are added as the last events.
The OVR_FLOW_M bit in Register 0x01 sets the mode of
operation during overflows. Clearing the OVR_FLOW_M bit
causes new incoming events to be discarded, and setting this bit
rolls over and overwrites old data with new data starting at the
first event.
